1. Field of the Invention
This invention relates to a method for fabricating a semiconductor device and more particularly to a method for forming isolation films of an oxide on a semiconductor substrate having one or more trenches and one or more islands bounded by the trenches.
2. Description of the Prior Art
Recently, circuit elements are formed not only on a main surface of a semiconductor substrate but also cn side walls of trenches cut into the main surface in order to provide a more highly integrated circuit device. Therefore, it is often necessary to form isolation films not only on the main surface but also on the side walls and bottom faces of the trenches.
FIGS. 1A to 1I are schematic fragmentary sectional views, illustrating the conventional steps for forming isolation oxide films on a silicon substrate having trenches and islands bounded by the trenches.
Referring to FIG. 1A, an oxidation mask layer 2 of silicon nitride is formed with an underlayer 3 of silicon oxide over an upper main surface of a silicon substrate 1.
Referring to FIG. 1B, the oxidation mask layer 2 is patterned by photolithography and then an isolation oxide film 4 is formed on the main surface by thermally oxidizing the silicon substrate 1. Thereafter, the patterned oxidation mask 2 is removed once and a net, oxidation mask layer 2 is again formed, covering the isolation film 4, over the entire main surface of the substrate 1.
Referring to FIG. 1C, an etching mask layer 5 of silicon oxide deposited on the new oxidation mask layer 2 is patterned by photolithography and then trenches are cut through the oxidation mask layer 2 and underlayer 3 into the substrate 1 by etching with the patterned etching mask 5.
Referring to FIG. 1D, the exposed silicon surfaces in the trenches of the substrate 1 are thermally oxidized and then the entire free surface over the substrate is covered with a further formed oxidation mask layer 2 of silicon nitride.
Referring to FIG. 1E, the oxidation mask layer 2 is patterned by etching with a multi-level resist which has been patterned by photolithography. In this figure, the oxidation mask layer 2 is removed in a side wall region of a trench where an isolation oxide film is to be formed, and the first-level resist 6 is shown as remained. A multi-level resist is described in detail by J. M. Moran et al. in J. Vac. Sci. Technol., 16(6), 1979, pp. 1620-1624.
Referring to FIG. 1F, after the multi-level resist 6 is removed, the entire free surface over the substrate is covered with a silicon oxide layer 7.
Referring to FIG. 1G, the silicon oxide layer 7 is subjected to an anisotropic etching such as a reactive ion etching from above, thereby to remove the oxidation mask layer 2 and underlayer 3 in the bottom regions of the trenches.
Referring to FIG. 1H, the silicon oxide layers 7 and 5 are etched away.
Finally referring to FIG. 1I, the silicon substrate 1 is thermally oxidized with the patterned oxidation mask 2. As a result, isolation oxide films 4 are formed not only in the island region but also in the side wall and bottom regions of the trenches.
In the above described conventional method, the isolation oxide films in the trench regions can not be formed in a self-aligned manner with respect to those in the island regions, because the different oxidation masks are used for forming the isolation oxide films in the island regions and the trench regions. Thus, marginal areas must be given to allow a small mismatch in the mask alignment according to the pattern layout.
Further, if an etching region for forming a trench is partly overlapped with an isolation oxide film 4 on the main surface of the substrate 1 because of a small mismatch in the mask alignment of the etching mask 5 (refer to FIG. 1C), the isolation oxide film 4 will cause oblique etching of the substrate 1 and make the neighboring side wall of the trench inclined. When the side wall of the trench is inclined, it is likely that the silicon oxide layer 7 formed on the inclined side wall is removed during the reactive ion etchig from above (refer to FIG. 1G).